The present invention relates to transition metal compounds and to polymerisation catalyst systems employing them.
The use of certain transition metal compounds to polymerise 1-olefins, for example, ethylene, is well established in the prior art. The use of Ziegler-Natta catalysts, for example, those catalysts produced by activating titanium halides with organometalric compounds such as triethylaluminium, is fundamental to many commercial processes for manufacturing polyolefins. Over the last twenty or thirty years, advances in the technology have led to the development of Ziegler-Natta catalysts which have such high activities that that olefin polymers and copolymers containing very low concentrations of residual catalyst can be produced directly in commercial polymerisation processes. The quantities of residual catalyst remaining in the produced polymer are so small as to render unnecessary their separation and removal for most commercial applications. Such processes can be operated by polymerising the monomers in the gas phase, or in solution or in suspension in a liquid hydrocarbon diluent. Polymerisation of the monomers can be carried out in the gas phase (the xe2x80x9cgas phase processxe2x80x9d), for example by fluidising under polymerisation conditions a bed comprising the target polyolefin powder and particles of the desired catalyst using a fluidising gas stream comprising the gaseous monomer. In the so-called xe2x80x9csolution processxe2x80x9d the (co)polymerisation is conducted by introducing the monomer into a solution or suspension of the catalyst in a liquid hydrocarbon diluent under conditions of temperature and pressure such that the produced polyolefin forms as a solution in the hydrocarbon diluent. In the xe2x80x9cslurry processxe2x80x9d the temperature, pressure and choice of diluent are such that the produced polymer forms as a suspension in the liquid hydrocarbon diluent. These processes are generally operated at relatively low pressures (for example 10-50 bar) and low temperature (for example 50 to 150xc2x0 C.).
Commodity polyethylenes are commercially produced in a variety of different types and grades. Homopolymerisation of ethylene with transition metal based catalysts leads to the production of so-called xe2x80x9chigh densityxe2x80x9d grades of polyethylene. These polymers have relatively high stiffness and are useful for making articles where inherent rigidity is required. Copolymerisation of ethylene with higher 1-olefins (eg butene, hexene or octene) is employed commercially to provide a wide variety of copolymers differing in density and in other important physical properties. Particularly important copolymers made by copolymerising ethylene with higher 1-olefins using transition metal based catalysts are the copolymers having a density in the range of 0.91 to 0.93. These copolymers which are generally referred to in the art as xe2x80x9clinear low density polyethylenexe2x80x9d are in many respects similar to the so called xe2x80x9clow densityxe2x80x9d polyethylene produced by the high pressure free radical catalysed polyrnerisation of ethylene. Such polymers and copolymers are used extensively in the manufacture of flexible blown film.
An important feature of the microstructure of the copolymers of ethylene and higher 1-olefins is the manner in which polymerised comonomer units are distributed along the xe2x80x9cbackbonexe2x80x9d chain of polymerised ethylene units. The conventional Ziegler-Natta catalysts have tended to produce copolymers wherein the polymerised comonomer units are clumped together along the chain. To achieve especially desirable film properties from such copolymers the comonomer units in each copolymer molecule are preferably not clumped together, but are well spaced along the length of each linear polyethylene chain. In recent years the use of certain metallocene catalysts (for example biscyclopentadienyizirconium dichloride activated with alumoxane) has provided catalysts with potentially high activity and capable of providing an improved distribution of the comonomer units. However, metallocene catalysts of this type suffer from a number of disadvantages, for example, high sensitivity to impurities when used with commercially available monomers, diluents and process gas streams, the need to use large quantities of expensive alumoxanes to achieve high activity, and difficulties in putting the catalyst on to a suitable support.
WO98/27124 discloses that ethylene may be polymerised by contacting it with certain iron or cobalt complexes of selected 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines). These complexes are disclosed as being suitable for preparing homopolymers of ethylene. Activities of from 6 to 2985 g/mmol/h/bar are shown.
We have developed novel catalysts utilising complexes such as those disclosed in WO 98/27124 which provide excellent activities and products. Accordingly in a first aspect the invention provides a catalyst for the polymerisation of olefins comprising
(1) a compound of the formula B 
wherein M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III] or Ru[IV]; X represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1, R2, R3, R4, R5, R6 and R7 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; and when any two or more of R1-R7 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents;
(2) an activator which is an alkylalumoxane; and
(3) additionally to (2), a compound of the formula AlR3, where each R is independently C1-C12 alkyl or halo.
We have found that the incorporation of component (3) into the catalyst can result in significant improvements in activity. The three substituents R in compound (3), which may be the same or different, are preferably hydrogen, methyl, ethyl, butyl or chloro. Preferred compounds (3) include trimethylaluminium (TMA), triethylaluminium (TEA), tri-isobutylaluminium (TIBA), tri-n-octylaluminium, ethylalumninium dichloride and diethylaluminium chloride. Most preferred are TMA and TIBA However the preferred compound (3) may depend on the polymerisation conditions in which the catalyst is employed: for example, TMA is particularly effective at improving catalyst activity in gas phase and also the activity of unsupported catalysts in slurry phase, whilst TIBA is particularly effective in slurry phase polymerisation generally.
As activator (2), the catalyst of the invention includes an alkylalumoxane which is normally a (C1-C4) alkylalumoxane, the alkyl group generally being methyl, ethyl, propyl or isobutyl. Preferred is methylalumoxane (also known as methylaluminoxane or MAO) or modified methylalumoxane (MMAO), which additionally contains isobutylalumoxane. The term xe2x80x9calkylalumoxanexe2x80x9d as used in this specification includes alkylalumoxanes available commercially which may contain a proportion, typically about 10 wt %, but optionally up to 50 wt %, of the corresponding trialkylaluminium; for instance, commercial MAO usually contains approximately 10 wt % trimethylaluminium (TMA), whilst commercial MMAO contains both TMA and TIBA. Quantities of alkylalumoxane quoted herein include such trialkylalkylaluminium impurities, and accordingly component (3) in this invention is considered to comprise compounds of the formula AlR3 additional to any AlR3 compound incorporated within the alkylalumoxane (2), and quantities of component (3) quoted herein are calculated on that basis.
In the preparation of the catalyst systems of the present invention the quantity of activating compound (2) to be employed is easily determined by simple testing, for example, by the preparation of small test samples which can be used to polymerise small quantities of the monomer(s) and thus to determine the activity of the produced catalyst. It is generally found that the quantity employed is sufficient to provide 0.1 to 20,000 atoms, preferably 1 to 2000 atoms of aluminium per Fe, Co, Mn or Ru metal atom in the compound of Formula B. The amount of activator (2) required for optimum performance may also depend on the amount of alkylaluminium compound (3) present. For example, when compound (3) is trimethyl aluminium (TMA), and the amount of TMA in the catalyst is less than 500 molar equivalents relative to the metal atom of compound (1), the amount of alkylalumoxane (usually MAO) is preferably at least 1000 molar equivalents. However if 500 equivalents or more of TMA are present, the optimum amount of alkylalumoxane (usually MAO) is from 500 to 1000 equivalents.
Preferably in Formula B above M is Fe[II], Fe[III], Ru[II], Ru[III] or Ru[IV]; X represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1, R2, R3, R4, R5, R6 and R7 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; and when any two or more of R1-R7 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents.
Further compounds for use in the present invention include those comprising the skeletal unit depicted in Formula Z: 
wherein M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III] or Ru[IV]; X represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1 to R4, R6 and R19 to R28 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; when any two or more of R1 to R4, R6 and R19 to R28 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents; with the proviso that at least one of R19, R20, R21 and R22 is hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl when neither of the ring systems P and Q forms part of a polyaromatic fused-ring system. In this particular aspect of the present invention, in the case that neither of the ring systems P and Q forms part of a polyaromatic ring system, it is preferred that at least one of R19 and R20, and at least one of R21 and R22 is selected from hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, and most preferably each of R19, R20, R21 and R22 is selected from hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl.
Subject to the foregoing provisos regarding R19, R20, R21 and R22 in Formula Z, R1 to R4, R6 and R19 to R28 in the compounds depicted in Formulae B and Z of the present invention are preferably independently selected from hydrogen and C1 to C8 hydrocarbyl, for example, methyl, ethyl, n-propyl, n-butyl, n-hexyl, and n-octyl. In Formula B, R5 and R7 are preferably independently selected from substituted or unsubstituted alicyclic, heterocyclic or aromatic groups, for example, phenyl, 1-naphthyl, 2-naphthyl, 2-methylphenyl, 2-ethylphenyl, 2,6-diisopropylphenyl, 2,3-diisopropylphenyl, 2,4-disopropylphenyl, 2,6-di-n-butylphenyl, 2,6-dimethylphenyl, 2,3-dimethylphenyl, 2,4-dimethylphenyl, 2-t-butylphenyl, 2,6-diphenylphenyl, 2,4,6-trimethylphenyl, 2,6-trifluoromethylphenyl, 4-bromo-2,6-dimethylphenyl, 3,5-dichloro2,6-diethylphenyl, and 2,6,bis(2,6-dimethylphenyl)phenyl, cyclohexyl and pyridinyl.
The ring systems P and Q in Formula Z are preferably independently 2,6-hydrocarbylphenyl or fused-ring polyaromatic, for example, 1-naphthyl, 2-naphthyl, 1-phenanthrenyl and 8-quinolinyl.
In the compound of Formula B and Z of the present invention, M is preferably Fe[II] or Co[II].
Yet further compounds suitable for the catalyst systems of the present invention are those comprising the skeletal unit depicted in Formula T: 
wherein M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III] or Ru[IV]; X represents an atom or group covalertly or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1 to R4, R6 and R29 to R32 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; when any two or more of R1 to R4, R6 and R29 to R32 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents.
In the compound of Formula B of the present invention, M is preferably Fe[II]. In the compounds of Formula Z or Formula T of the present invention, M is preferably Fe[II], Mn[II] or Co[II].
Examples of the atom or group X in the compounds of Formula B, Z and T are halide, for example, chloride, bromide; hydride; hydrocarbyloxide, for example, methoxide, ethoxide, isopropoxide, phenoxide; carboxylate, for example, formate, acetate, benzoate; hydrocarbyl, for example, methyl, ethyl, propyl, butyl, octyl, decyl, phenyl, benzyl; substituted hydrocarbyl; heterohydrocarbyl; tosylate; and triflate. Preferably X is selected from halide, hydride and hydrocarbyl. Chloride is particularly preferred.
The following are examples of nitrogen-containing transition metal complexes that can be employed in the catalyst of the present invention:
2,6-diacetylpyridinebis(2,6-diisopropylanil)FeCl2 
2,6-diacetylpyridine(2,6-diisopropylanil)MnCl22,6-diacetylpyridine(2,6-diisopropylanil)CoCl2 
2,6-diacetylpyridinebis(2-tert.-butylanil)FeCl2 
2,6-diacetylpyridinebis(2,3-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2-methylanil)FeCl2 
2,6-diacetylpyridinebis(2,4-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2,6-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2,4,6 trimethyl anil)FeCl2 
2,6-dialdiminepyridinebis(2,6-dimethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-diethylanil)FeCl2 
2,6dialdiminepyridinebis(2,6-diisopropylanil)FeCl2 
2,6-dialdiminepyridinebis(1-naphthil)FeCl2 and
2,6-bis(1,1-diphenylhydrazone)pyridine.FeCl2.
A preferred complex of the present invention is 2,6-diacetylpyridinebis(2,4,6 trimethyl anil)FeCl2.
In a further aspect of the present invention the catalyst system may additionally comprise (4) a neutral Lewis base. Neutral Lewis bases are well known in the art of Ziegler-Natta catalyst polymerisation technology. Examples of classes of neutral Lewis bases suitably employed in the present invention are unsaturated hydrocarbons, for example, alkenes or alkynes, primary, secondary and tertiary amines, amides, phosphoramides, phosphines, phosphites, ethers, thioethers, nitrites, carbonyl compounds, for example, esters, ketones, aldehydes, carbon monoxide and carbon dioxide, sulphoxides, sulphones and boroxines. Although 1-olefins are capable of acting as neutral Lewis bases, for the purposes of the present invention they are regarded as monomer or comonomer 1-olefins and not as neutral Lewis bases per se. However, alkenes which are internal olefins, for example, 2-butene and cyclohexene are regarded as neutral Lewis bases in the present invention. Preferred Lewis bases are tertiary amines and aromatic esters, for example, dimethylaniline, diethylaniline, tributylamine, ethylbenzoate and benzylbenzoate. In this particular aspect of the present invention, components (1), (2) and (4) of the catalyst system can be brought together simultaneously or in any desired order. However, if components (2) and (4) are compounds which interact together strongly, for example, form a stable compound together, it is preferred to bring together either components (1) and (2) or components (1) and (4) in an initial step before introducing the final defined component. Preferably components (1) and (4) are contacted together before component (2) is introduced. The quantities of components (1) and (2) employed in the preparation of this catalyst system are suitably as described above in relation to the catalysts of the present invention. The quantity of the neutral Lewis Base [component (4)] is preferably such as to provide a ratio of component (1):component (4) in the range 100:1 to 1:1000, most preferably in the range 1:1 to 1:20. Components (1), (2) and (4) of the catalyst system can brought together, for example, as the neat materials, as a suspension or solution of the materials in a suitable diluent or solvent (for example a liquid hydrocarbon), or, if at least one of the components is volatile, by utilising the vapour of that component. The components can be brought together at any desired temperature. Mixing the components together at room temperature is generally satisfactory. Heating to higher temperatures eg up to 120xc2x0 C. can be carried out if desired, eg to achieve better mixing of the components. It is preferred to carry out the bringing together of components (1), (2) and (4) in an inert atmosphere (eg dry nitrogen) or in vacua. If it is desired to use the catalyst on a support material (see below), this can be achieved, for example, by preformiing the catalyst system comprising components (1), (2) and (4) and impregnating the support material preferably with a solution thereof, or by introducing to the support material one or more of the components simultaneously or sequentially. If desired the support material itself can have the properties of a neutral Lewis base and can be employed as, or in place of component (4). An example of a support material having neutral Lewis base properties is poly(aminostyrene) or a copolymer of styrene and aminostyrene (ie vinylaniline).
The compounds employed as catalysts in the catalyst systems of the present invention can if desired comprise more than one of the above-defined transition metal compounds. The catalyst may comprise, for example a mixture of 2,6-diacetylpyridinebis(2,6-diisopropylanil)FeCl2 complex and 2,6-diacetylpyridinebis (2,4,6-trimethylanil)FeCl2 complex, or a mixture of 2,6-diacetylpyridine(2,6-diisopropylanil)CoCl2 and 2,6-diacetylpyridinebis(2,4,6-trimethylanil)FeCl2. In addition to said one or more defined transition metal compounds, the catalysts can also include one or more other types of transition metal compounds or catalysts, for example, transition metal compounds of the type used in conventional Ziegler-Natta catalyst systems, metallocene-based catalysts, or heat activated supported chromium oxide catalysts (eg Phillips-type catalyst).
The catalysts employed in the present invention can be unsupported or supported on a support material, for example, silica, alumina, or zirconia, or on a polymer or prepolymer, for example polyethylene, polystyrene, or poly(aminostyrene). If desired the catalysts can be formed in situ in the presence of the support material, or the support material can be pre-impregnated or premixed, simultaneously or sequentially, with one or more of the catalyst components. The catalysts can if desired be supported on a heterogeneous catalyst, for example, a magnesium halide supported Ziegler Natta catalyst, a Phillips type (chromium oxide) supported catalyst or a supported metallocene catalyst. Formation of the supported catalyst can be achieved for example by treating the transition metal compounds with alumoxane in a suitable inert diluent, for example a volatile hydrocarbon, slurrying a particulate support material with the product and evaporating the volatile diluent. The quantity of support material employed can vary widely, for example from 100,000 to 1 grams per gram of metal present in the transition metal compound.
A further aspect of the invention comprises the use of a compound of the formula AlR3, where each R is independently C1-C12 alkyl or halo, to enhance the catalytic activity in polymerisation of 1-olefins of a compound of the Formula B as defined above.
The present invention further provides a process for the polymerisation and copolymerisation of 1-olefins comprising contacting the monomeric olefin under polymerisation conditions with a polymerisation catalyst system comprising
(1) a compound of the formula B as defined above
(2) an alkylalumoxane activator
(3) additionally to (2), a compound of the formula AlR3, where each R is independently C1-C12 alkyl or halo.
In a preferred process the catalyst compound (1) is activated with the alkylalumoxane (2) before contact with the monomer to be polymerised.
The polymerisation conditions can be, for example, solution phase, slurry phase or gas phase. If desired, the catalyst system can be used to polymerise ethylene under high pressure/high temperature process conditions wherein the polymeric material forms as a melt in supercritical ethylene. Preferably the polymerisation is conducted under gas phase fluidised bed conditions. Slurry phase polymerisation conditions or gas phase polymerisation conditions are particularly useful for the production of high density grades of polyethylene. In these processes the polymerisation conditions can be batch, continuous or semi-continuous. In the slurry phase process and the gas phase process, the catalyst is generally fed to the polymerisation zone in the form of a particulate solid. This solid can be, for example, an undiluted solid catalyst system formed from a nitrogen-containing complex and an activator, or can be the solid complex alone. In the latter situation, the activator can be fed to the polymerisation zone, for example as a solution, separately from or together with the solid complex. Preferably the catalyst system or the transition metal complex component of the catalyst system employed in the slurry polymerisation and gas phase polymeriastion is supported on a support material. Most preferably the catalyst system is supported on a support material prior to its introduction into the polymerisation zone. Suitable support materials are, for example, silica, alumina, zirconia, talc, kieselguhr, or magnesia. Impregnation of the support material can be carried out by conventional techniques, for example, by forming a solution or suspension of the catalyst components in a suitable diluent or solvent, and slurrying the support material therewith. The support material thus impregnated with catalyst can then be separated from the diluent for example, by filtration or evaporation techniques.
In the slurry phase polymerisation process the solid particles of catalyst, or supported catalyst, are fed to a polymerisation zone either as dry powder or as a slurry in the polymerisation diluent. Preferably the particles are fed to a polymerisation zone as a suspension in the polymerisation diluent. The polymerisation zone can be, for example, an autoclave or similar reaction vessel, or a continuous loop reactor, e.g. of the type well-known in the manufacture of polyethylene by the Phillips Process. When the polymerisation process of the present invention is carried out under slurry conditions the polymerisation is preferably carried out at a temperature above 0xc2x0 C., most preferably above 15xc2x0 C. The polymerisation temperature is preferably maintained below the temperature at which the polymer commences to soften or sinter in the presence of the polymerisation diluent. If the temperature is allowed to go above the latter temperature, fouling of the reactor can occur. Adjustment of the polymerisation within these defined temperature ranges can provide a useful means of controlling the average molecular weight of the produced polymer. A further useful means of controlling the molecular weight is to conduct the polymerisation in the presence of hydrogen gas which acts as chain transfer agent. Generally, the higher the concentration of hydrogen employed, the lower the average molecular weight of the produced polymer.
The use of hydrogen gas as a means of controlling the average molecular weight of the polymer or copolymer applies generally to the polymerisation process of the present invention. For example, hydrogen can be used to reduce the average molecular weight of polymers or copolymers prepared using gas phase, slurry phase or solution phase polymerisation conditions. The quantity of hydrogen gas to be employed to give the desired average molecular weight can be determined by simple xe2x80x9ctrial and errorxe2x80x9d polymerisation tests.
The polymerisation process of the present invention provides polymers and copolymers, especially ethylene polymers, at remarkably high productivity (based on the amount of polymer or copolymer produced per unit weight of nitrogen-containing transition metal complex employed in the catalyst system). This means that relatively very small quantities of transition metal complex are consumed in commercial processes using the process of the present invention. It also means that when the polymerisation process of the present invention is operated under polymer recovery conditions that do not employ a catalyst separation step, thus leaving the catalyst, or residues thereof, in the polymer (eg as occurs in most commercial slurry and gas phase polymerisation processes), the amount of transition metal complex in the produced polymer can be very small. Experiments carried out with the catalyst of the present invention show that, for example, polymerisation of ethylene under slurry polymerisation conditions can provide a particulate polyethylene product containing catalyst so diluted by the produced polyethylene that the concentration of transition metal therein falls to, for example, 1 ppm or less wherein xe2x80x9cppmxe2x80x9d is defined as parts by weight of transition metal per million parts by weight of polymer. Thus polyethylene produced within a polymerisation reactor by the process of the present invention may contain catalyst diluted with the polyethylene to such an extent that the transition metal content thereof is, for example, in the range of 1-0.0001 ppm, preferably 1-0.001 ppm. Using a catalyst comprising a nitrogen-containing Fe complex in accordance with the present invention in, for example, a slurry polymerisation, it is possible to obtain polyethylene powder wherein the Fe concentration is, for example, 1.03 to 0.11 parts by weight of Fe per million parts by weight of polyethylene.
Suitable monomers for use in the polymerisation process of the present invention are, for example, ethylene, propylene, butene, hexene, methyl methacrylate, methyl acrylate, butyl acrylate, acrylonitrile, vinyl acetate, and styrene. Preferred monomers for homopolymerisation processes are ethylene and propylene. The catalyst can also be used for copolymerising ethylene with other 1-olefins such as propylene, 1-butene, 1-hexene, 4-methylpentene-1, and octene.
Methods for operating gas phase polymerisation processes are well known in the art. Such methods generally involve agitating (e.g. by stirring, vibrating or fluidising) a bed of catalyst, or a bed of the target polymer (i.e. polymer having the same or similar physical properties to that which it is desired to make in the polymerisation process) containing a catalyst, and feeding thereto a stream of monomer at least partially in the gaseous phase, under conditions such that at least part of the monomer polymerises in contact with the catalyst in the bed. The bed is generally cooled by the addition of cool gas (e.g. recycled gaseous monomer) and/or volatile liquid (e.g. a volatile inert hydrocarbon, or gaseous monomer which has been condensed to form a liquid). The polymer produced in, and isolated from, gas phase processes forms directly a solid in the polymerisation zone and is free from, or substantially free from liquid. As is well known to those skilled in the art, if any liquid is allowed to enter the polymerisation zone of a gas phase polymerisation process the quantity of liquid is small in relation to the quantity of polymer present in the polymerisation zone. This is in contrast to xe2x80x9csolution phasexe2x80x9d processes wherein the polymer is formed dissolved in a solvent, and xe2x80x9cslurry phasexe2x80x9d processes wherein the polymer forms as a suspension in a liquid diluent.
The gas phase process can be operated under batch, semi-batch, or so-called xe2x80x9ccontinuousxe2x80x9d conditions. It is preferred to operate under conditions such that monomer is continuously recycled to an agitated polymerisation zone containing polymerisation catalyst, make-up monomer being provided to replace polymerised monomer, and continuously or intermittently withdrawing produced polymer from the polymerisation zone at a rate comparable to the rate of formation of the polymer, fresh catalyst being added to the polymerisation zone to replace the catalyst withdrawn form the polymerisation zone with the produced polymer.
In the preferred embodiment of the gas phase polymerisation process of the present invention, the gas phase polymerisation conditions are preferably gas phase fluidised bed polymerisation conditions.
Methods for operating gas phase fluidised bed processes for making polyethylene and ethylene copolymers are well known in the art. The process can be operated, for example, in a vertical cylindrical reactor equipped with a perforated distribution plate to support the bed and to distribute the incoming fluidising gas stream through the bed. The fluidising gas circulating through the bed serves to remove the heat of polymerisation from the bed and to supply monomer for polymerisation in the bed. Thus the fluidising gas generally comprises the monomer(s) normally together with some inert gas (eg nitrogen) and optionally with hydrogen as molecular weight modifier. The hot fluidising gas emerging from the top of the bed is led optionally through a velocity reduction zone (this can be a cylindrical portion of the reactor having a wider diameter) and, if desired, a cyclone and or filters to disentrain fine solid particles from the gas stream. The hot gas is then led to a heat exchanger to remove at least part of the heat of polymerisation. Catalyst is preferably fed continuously or at regular intervals to the bed. At start up of the process, the bed comprises fluidisable polymer which is preferably similar to the target polymer. Polymer is produced continuously within the bed by the polymerisation of the monomer(s). Preferably means are provided to discharge polymer from the bed continuously or at regular intervals to maintain the fluidised bed at the desired height. The process is generally operated at relatively low pressure, for example, at 10 to 50 bars, although it may be operated at pressures from 10 to 100 bars; and at temperatures for example, between 50 and 120 xc2x0 C. The temperature of the bed is maintained below the sintering temperature of the fluidised polymer to avoid problems of agglomeration.
In the gas phase fluidised bed process for polymerisation of olefins the heat evolved by the exothermic polymerisation reaction is normally removed from the polymerisation zone (ie, the fluidised bed) by means of the fluidising gas stream as described above. The hot reactor gas emerging from the top of the bed is led through one or more heat exchangers wherein the gas is cooled. The cooled reactor gas, together with any make-up gas, is then recycled to the base of the bed. In the gas phase fluidised bed polymerisation process of the present invention it is desirable to provide additional cooling of the bed (and thereby improve the space time yield of the process) by feeding a volatile liquid to the bed under conditions such that the liquid evaporates in the bed thereby absorbing additional heat of polymerisation from the bed by the xe2x80x9clatent heat of evaporationxe2x80x9d effect. When the hot recycle gas from the bed enters the heat exchanger, the volatile liquid can condense out. In one embodiment of the present invention the volatile liquid is separated from the recycle gas and reintroduced separately into the bed. Thus, for example, the volatile liquid can be separated and sprayed into the bed. In another embodiment of the present invention the volatile liquid is recycled to the bed with the recycle gas. Thus the volatile liquid can be condensed from the fluidising gas stream emerging from the reactor and can be recycled to the bed with recycle gas, or can be separated from the recycle gas and sprayed back into the bed.
The method of condensing liquid in the recycle gas stream and returning the mixture of gas and entrained liquid to the bed is described in EP-A-0089691 and EP-A-0241947. It is preferred to reintroduce the condensed liquid into the bed separate from the recycle gas using the process described in our U.S. Pat. No. 5,541,270, the teaching of which is hereby incorporated into this specification.
When using the catalysts of the present invention under gas phase polymerisation conditions, the catalyst, or one or more of the components employed to form the catalyst can, for example, be introduced into the polymerisation reaction zone in liquid form, for example, as a solution in an inert liquid diluent. Thus, for example, the transition metal component, or the activator component, or both of these components can be dissolved or slurried in a liquid diluent and fed to the polymerisation zone. Under these circumstances it is preferred the liquid containing the component(s) is sprayed as fine droplets into the polymerisation zone. The droplet diameter is preferably within the range 1 to 1000 microns. EP-A-0593083, the teaching of which is hereby incorporated into this specification, discloses a process for introducing a polymerisation catalyst into a gas phase polymerisation. The methods disclosed in EP-A-0593083 can be suitably employed in the polymerisation process of the present invention if desired.